clk55 amg battery/alternator visit workshop problem - Mercedes-Benz Forum

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Old 08-03-2013, 01:22 PM   #1 (permalink)
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Date registered: Oct 2009
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clk55 amg battery/alternator visit workshop problem

As some of you may know ive recently brought a CLK55 which was off of the road for a few years and im now using it as my daily driver.

When i brought it the unbranded aftermarket alternator had failed and was replaced with a new high quality aftermarket unit as i was unable to get a genuine bosch unit for a reasonable price.

All was well for about 3 weeks and about a week ago i was driving on the motorway and the "battery/alternator light visit workshop" message appeared. I found that if i dropped my rpm below 2000 rpm the light would extinguish and if i raised my rpm above 2000rpm for a few seconds it would reappear.

I removed the alternator and sent it to be tested. It came back as working fine with no problems. I re-installed it and its exactly the same.

I connected my multi meter to the output on the back of the alternator and alternator body to read the voltage it produces. On ldle it produces 13.8 volts but when i hold the rpms up it starts to creep down at about 0.3 volts a second until the warning appears.

As soon as i go back to idling it jumps back to 13.8 volts and the warning goes away. Does this sound like a voltage regulator?
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Old 08-03-2013, 01:46 PM   #2 (permalink)
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Originally Posted by scrappyloz View Post

As soon as i go back to idling it jumps back to 13.8 volts and the warning goes away. Does this sound like a voltage regulator?
As you have eiminated the alternator, this does sound like the VR is mis-behaving at higher voltage inputs. They are not very expensive.

Why wasn't the alternator and VR tested together? It could be an electronic fault or bad brushes.
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Old 08-04-2013, 08:42 AM   #3 (permalink)
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They were both tested according to the receipt but i guess not properly
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Old 08-11-2013, 03:52 AM   #4 (permalink)
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Voltage regulator replaced and still misbehaving but not as bad. What is odd is i have been driving the car everyday using all the options (lights heated sets radio etc) and the car is showing no problems related to battery or charging problems.

What senses the voltage off of the alternator to turn the warning message on and off? is it a sam unit and is there any way of inspecting it.
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Old 08-11-2013, 05:33 PM   #5 (permalink)
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A visit to the dealer parts department might have revealed an open campaign to replace the alternator and harness under warranty. The campaign is still open but I am not sure about the years.
My 2002 was repaired in 05/06. It sounds like the regulator or a wire problem as the engine should increase the battery voltage to 14.1 -14.3. Confirm clean battery connections and all battery cells are strong.

Hope this helps,

Mike
PS: stay away from URO electrical parts

Last edited by Mercedes Mike; 08-11-2013 at 05:35 PM. Reason: added content
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Old 08-14-2013, 11:30 AM   #6 (permalink)
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It's also possible you have a damaged commutator or winding or a weak diode connection. As the revs increase and things are moving faster, it fails.

When they test it, it's just sitting on a bench at mild speed, so it's never going to show up. Either disassemble and inspect and test the components, or replace the alternator. (That it seemed to improve after the new regulator would cause me to first inspect the commutators, but I'd still be tearing it down to check and test everything.)

Good luck.
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Old 08-14-2013, 11:31 AM   #7 (permalink)
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A visit to the dealer parts department might have revealed an open campaign to replace the alternator and harness under warranty. The campaign is still open but I am not sure about the years.
My 2002 was repaired in 05/06...

Hope this helps,

Mike
This is a good thought, but I don't think the campaign extended to the UK. Many times such things are country-specific.
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Old 08-20-2013, 01:56 PM   #8 (permalink)
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Alternator has been returned to the supplier for a full overhaul so fingers crossed it will back and working in a few days
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